ovsdb: Use '.' as field separator.
ovsdb: Use '.' as field separator.

A previous patch saved ovs interface external_ids as node metadata
with the form "ExtID:<name".  We had actually discussed using a '.' as
the separator, not ':', so change it.

Change-Id: I0e4aa36fd005030eccaad6686896aacdd9aba5ec
Signed-off-by: Russell Bryant <b61ba72562ef1f6718e4f653428a87ed07b576b8@redhat.com>

redhat-cip/skydive,redhat-cip/skydive,lebauce/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,redhat-cip/skydive,safchain/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,safchain/skydive